Yes, ASTERISMS is a Scrabble word!

Letter values in Scrabble US: A 1, S 1, T 1, E 1, R 1, I 1, S 1, M 3, S 1.

Is it a valid word?

ASTERISMS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

(mineralogy) a star-shaped figure with six rays that is seen in some crystal structures under reflected or transmitted light
(astronomy) a cluster of stars (or a small constellation)